This invention concerns a method to feed tuyxc3xa8res in electric furnaces, and the relative feed device, as set forth in the respective main claims.
The invention is applied in electric melting furnaces, and particularly in electric arc furnaces, to send a comburent mixture to the bottom tuyxc3xa8res provided in the hearth of the furnace.
The feed method according to the invention makes it possible to prolong the working life of the tuyxc3xa8res and improve the productivity thereof, reducing wear and deterioration and optimising efficiency.
The invention also makes it possible to increase the effect of the heat contribution which derives from the combustion of the substances inside the furnace and the comburent mixture delivered through the tuyxc3xa8res.
In electric melting furnaces, whether they be fed with direct or alternating current, the state of the art covers the use of tuyxc3xa8res on the bottom through which gassy substances with a high comburent power are introduced inside the furnace, with the function of determining and intensifying combustion reactions with the chemical substances which are liberated inside the furnace.
These combustion reactions make it possible to recover the intrinsic potential of energy possessed mainly by carbon and other chemical substances liberated inside the furnace by means of an exothermic oxidation reaction.
These combustion reactions supply an auxiliary contribution of energy with respect to the main means which the furnace, thus allowing a great saving in energy, speeding up the melting cycles and improving the quality of the metal produced.
The tuyxc3xa8res normally comprise a conduit, with its mouth protruding inside the furnace, connected to means to feed the comburent substance, normally oxygen or air enriched with oxygen.
Other gassy substances may also be sent through this conduit, such as azote, neutral gases or other similar substances, but one gas must always be separate from the other.
The gassy substance sent through the tuyxc3xa8res also has the function of stirring the bath of liquid metal.
In order to reduce wear on the upper part of the tuyxc3xa8re which protrudes into the furnacexe2x80x94wear caused by the enormous overheating due to the combustion reactionsxe2x80x94the state of the art provides to send a cooling gas, normally a mixture of hydrocarbons, azote, inert gases or similar, together with the comburent gas.
There have been proposals for tuyxc3xa8res wherein, peripherally to a central conduit which delivers the comburent substance, an annular conduit is provided which introduces the cooling mixture.
The present Applicant, in the European Patent application EP-A-625685, described a tuyxc3xa8re wherein the oxygen-delivery conduit is made of copper and is obtained by making a central hole in a polygonal or star-shaped profile, the slits obtained at the side of the central hole being used to send the cooling mixture.
The said patent application also describes how the melting cycle is divided into a plurality of intervals, each of which is characterised by specific parameters of pressure both of the comburent oxygen and also of the cooling mixture.
Moreover, the feed method of EPxe2x80x2685 provides to regulate the pressure of the oxygen by using suitable readers and transducers on the line of feed.
The consequent variations in the flow of oxygen influence the flow of at least one of the components of the cooling mixture, the flow of the other component or components of the cooling mixture consequently varying so as to ensure a constant and controlled feed to the mixing device which supplies the cooling mixture to the annular delivery conduits inside the furnace.
By using this adjustment technique, together with the particular conformation of the tuyxc3xa8res, their size and their specific distribution on the bottom of the furnace, excellent results have been obtained in terms of energy efficiency and the protection of the tuyxc3xa8res from wear and deterioration.
However, in practice, it has been seen that in the event that heavy material of a certain thickness is deposited above the outlet mouth of the tuyxc3xa8res, the comburent gassy substance is not able to perforate this and therefore the tuyxc3xa8re rapidly deteriorates and is ruined.
This problem means that it is soon necessary to carry out maintenance operations or to replace the tuyxc3xa8res, which means interrupting the cycle, long down-times and a great deal of work for the workers.
These problems also cause periods when the tuyxc3xa8res are working at low efficiency, with a consequent reduction in the energy supply and a deterioration in the quality of the molten metal produced.
The present Applicant has designed, tested and embodied this invention to overcome the shortcomings of the state of the art, and to obtain further advantages.
The invention is set forth and characterised in the respective main claims, while the dependent claims describe variants of the idea of the main embodiment.
According to the invention, the central conduit of each tuyxc3xa8re is fed with a mixture consisting of oxygen and a neutral gas such as azote or similar.
These two components are sent to a mixing device through relative lines comprising at least means to modulate the sending pressure.
According to a variant, the line to feed the neutral gas comprises at least two send lines each of which defines a specific level of pressure at inlet.
The second send line makes it possible to obtain an oxygen-neutral gas mixture under pressure.
According to the invention, the mixing device which feeds the central conduit of the tuyxc3xa8re is of the type which is suitable to create an increase in the speed of the gas at inlet, so that at outlet the jet of the mixture has a kinetic energy able to perforate at least partly any possible layer of material which has accumulated on the bottom.
According to a variant, the mixing device is of the type which is suitable to induce a high quantity of vortical motion in the mixture of comburent substances at outlet.
The present Applicant has verified that by using the mixture with the aforesaid components, and using a mixing device which develops a high speed and in the gases at outlet, it is possible to prolong the working life of the tuyxc3xa8res.
According to another variant, the feed line of the cooling mixture provides at least a mixing system of a type which will achieve high speed and vortical motion at outlet.